How to Work with the Example Code
The complete source code for the project you'll work on is available as a GitHub repository at https://github.com/miguelgrinberg/react-microblog.
My suggestion is that you type or copy the application code on your own, using the instructions I provide, at least for the first few chapters. I would recommend that you don't rely too much on the code that is on GitHub, as I think it is important that you familiarize yourself with the task of coding the React application, since this is what you will be doing when you work on your own projects.
However, the GitHub repository can be an invaluable reference if you get stuck and can't get the application to work on your own. In the README.md file of the repository you will find links to the specific code changes covered in each chapter.
For most web applications, some functionality related to data persistence can only be implemented by a back end service (sometimes also called an API). This book does not cover back end development, but I have created a companion back end application that you will need to set up and run to support your work with React. The back end application is also available on a GitHub repository, at https://github.com/miguelgrinberg/microblog-api, in case you are interested in reviewing it. When the time comes, you will be given instructions on how to get this project up and running.